1) Field of the Invention
The present invention relates to a system and method for separating or fractionating particles according to one or more physical criteria. The same system can be used to insert particles into another flow stream (a form of mixing).
2) Description of Related Art
A variety of fractionation schemes exist, ranging from gel-electrophoresis, capillary electrophoresis, and analytical centrifuging to novel, entropic barriers. Examples of these are described by J. Han, H. G. Craighead, Science 288, 1026-1029 (May 12, 2000) and D. Nykypanchuk, H. H. Strey, D. A. Hoagland, Science 297, 987-990 (Aug. 9, 2002). The majority of these known techniques separate a polydisperse mixture into bands containing particles that travel at different velocities along the direction of flow. This typically leads to batch processing. In electrophoresis a gel may be used to obtain a size-dependent mobility. Recovery of fractions is achieved through post-processing of the gel. However, despite its widespread use and effectiveness this methodology is slow and importantly, due to limited pore sizes, has difficulty in separating objects at the microscopic size level, for example cells, chromosomes, and colloidal matter.
Lithographically fabricated two-dimensional, asymmetric artificial gels are also used. Examples of these are described in the articles by D. Ertas, Physical Review Letters 80, 1548-1551 (Feb. 16, 1998); T. A. J. Duke, R. H. Austin, Physical Review Letters 80, 1552-1555 (Feb. 16, 1998) and C. F. Chou et al., Biophysical Journal 83, 2170-2179 (October, 2002). These gels yield separation transverse to the direction of flow. Because of this, they can be operated in a continuous fashion, with various fractions taken up by separate collection channels. However, sorting based on diffusion becomes impractically slow at the microscopic scale and above.
Another fractionation scheme that has been proposed is described in the article “Kinetically Locked-in Colloidal Transport in an Array of Optical Tweezers” by Korda et al, Physical Review Letters, Vol 89, Number 12, 16 September 2002. In this case, a monolayer of colloidal spheres is allowed to flow through an array of discrete optical traps. By varying the orientation of the trap lattice it was shown that the direction of flow of the spheres could be varied. Because of this, it was suggested that the lattice could be used to continuously fractionate mesoscopic particles. However, because of the use of a lattice of localized discrete traps, the observed kinetically locked-in channeling along low-index lattice vectors was intrinsically limited to small-angle deflections. In practice, this limits the practicality of the lattice for use in fractionation.
Fractionation systems are used in many different applications. One field where their use is becoming of increasing interest is that of microfluidics. In microfluidics, flow is predominantly laminar, creating challenges in the design of actuators such as mixers and sorters. The ability to select and sort both colloidal and biological matter in a manner related to its physical properties in a fast and efficient manner is a key requirement at this level.